Tenuous Support of a truss?

[JAE]

I've got a detail that I was given to consider for a roof truss support condition. Just want you to offer some opinions on its appropriateness.

It is a flat roof (1/4"/ft slope) with triple wood trusses (parallel chords) spanning about 31 feet and spaced at 6 feet o.c. The detail calls for these triple trusses to be supported by a 2x12 ledger board that is nailed into a supporting stud wall. Roof DL is about 18 psf. Roof Snow is about 25 psf.

The detail shows a large number of nails (7- 10d) used to connect the ledger to the 2x6 stud wall. I think this is an excessive number of nails and 7 nails from a 2x12 into one 2x6 stud edge seems like we'd have split wood. This ledger carries the truss load - and specifically with trusses at 6 feet o.c. a lot of truss load might go into one ledger-stud connection.

Any opinions? Here is the detail with the large arrow showing the critical note.

 

[UcfSE]

I agree with your concern about splitting the wood. That sounds like too many nails into an edge. Can you put a nailer against each side of the stud at the ledger location to have more "meat" to nail to?

 

[JAE]

Well, I was thinking about sitting the trusses ON the wall, and then running 2x6's up the sides of the truss ends to form the parapet - avoiding eccentric loads on the studs...I just couldn't believe that this connection was even safe and was looking for confirmation of my humble opinion.

 

[UcfSE]

It would seem that even if the studs at 16" o.c. could handle the load, I think you will have a hard time distributing the load to each stud through a single 2x ledger rather than having a godzilla point load on one stud wherever the truss lands. The ledger probably isn't stiff enough to distribute the load that well. I'd probably have a built-up column at each truss and then studs in between. The BUC can be sized to handle the eccentricty and still be continuous to make parapet framing work out. That shouldn't be that hard at 6 feet centers, maybe use a double ledger and have a double leg at the end of the truss so that it could bear on the ledger instead of bearing on a lg bucket.

 

[FSS]

Distributing load evenly will be a problem. Trusses are spaced too far apart to assume uniform load transfer, especially with single 2x10 ledger. Recommend putting multiple studs at each truss location and nailing hanger through the ledger into studs.

 

[mkberry]

I have 2 ideas, both with some drawbacks -
1. Bear the truss on top of the wall and have the parapet be built into the truss. This may require a triple top plate, or studs located directly below each truss depending on your load.
2. Provide (2) 2x10 blocking between studs at the truss locations. connect each end of the blocking to the adjacent studs with concealed flange hangers, and use another hanger to connect the truss to the blocking. If the truss hanger is close to the blocking hanger, the nails may interfere.

 

[haynewp]

I don't do much wood design but my first thought would be to support the trusses on individual columns in the wall and use an end column cap connector something like this:

http://www.strongtie.com/products/connectors/ecc-eccu.html

Then the studs can just be installed the way they are shown now at the sides of the columns. I would also tie the column into the shearwall so it can't kick out from under the truss, probably also connect it at the top inside face so it can't roll either.

Are the diaphragm loads going through the studs to get to the exterior wall sheathing? It may be done all the time but the few times I have had a load bearing stud wall with a parapet I remember trying to avoid that.

 

[haynewp]

Also I wonder what is supposed to be the continuous diaph. chord in the detail shown?

If the blocking was intended to transfer the shear from the roof into the wall it doesn't look very well placed.

 

[JAE]

Appreciate all the comments - when I first saw this detail it appeared OK until I really started digesting it -

What I think I will do is generally what a lot of you implied: run the truss OVER the wall, build up wood columns within the wall (comprised of multiple 2x6's); have a double top plate running down the wall directly under the truss bottom chord; extend other 2x6 studs up from the double top plate, past the top chord to form the parapet.

This gets the truss reaction centered on the supporting wall (wall/columns) and still allows for a parapet. The chord of the diaphragm would have to be a double 2x6 running along the top chord of the truss, directly inside of the parapet - we could provide a notch in the top chord at that point to allow this chord to be flush with the top of the truss.

Thanks again....